filter flagged xml elements (retry)

(This is a retry of Iecd32f85b25e)

This makes the xml parser skip over xml elements behind disabled flags

- [omakoto] The AconfigFlag class doesn't work on Ravenwood. I've disabled
the "useLayoutReadwrite" path for now, only on ravenwood.

This means some of your new tests may not pass on Ravenwood. Please add
the following annotation to skip such tests on Ravenwood.
@android.platform.test.annotations.DisabledOnRavenwood(bug = 396458006)

Bug:377974898
Bug: 396458006
Test: new cts FlaggedResourcesRWTest
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh -s

Flag: android.content.res.layout_readwrite_flags
Change-Id: I0f0aa7d8499247de9ed0efbb472e55a7ddd8e8b3